In physics class, the girl to boy ratio is $8$ to $2$. If there are a total of $20$ students, how many girls are there?
Explanation: A ratio of $8$ girls to $2$ boys means that a set of $10$ students will have $8$ girls and $2$ boys. A class of $20$ students has $2$ sets of $10$ students. Because we know that there are $8$ girls in each set of $10$ students, the class must have $2$ groups of $8$ girls each. There is a total of $16$ girls in physics class.